As described in U.S. patent application Ser. No. 13/647,838 entitled Process for Treating Fly Ash and a Rotary Mill Therefore by W. Clinton Pike filed Oct. 9, 2012 and incorporated herein by reference, activated fly ash has been utilized to replace as much as 50-80% by weight of Portland cement in the manufacture of concrete. In order to be able to activate the fly ash, a specialized rotary mill is utilized which intergrinds the fly ash with a calcium additive to expand the fly ash surface area to activate it so that it can be used to replace Portland cement in the manufacture of concrete.
The process described above has been exceptionally successful in the manufacture of high strength concrete, and as a result fly ash or pozzolan has provided an extremely inexpensive substitute for Portland cement in such applications.
As is well known, in order for concrete to be usable in areas of the country where the temperature oscillates from freezing to thawing, it is important that the entrained air in the mixture be in excess of 4%. The entrainment of the air protects the concrete against cracking when the temperature goes below freezing as well as when there is a freeze/thaw cycle.
However, it has been found that when activated carbon is injected to remove mercury from the flue gas of power plants, the resulting pozzolan or fly ash is contaminated with a certain amount of this injected activated carbon (ACI). When one attempts to activate the fly ash contaminated with activated carbon in the process described what happens is that air entrainment drops close to zero due to the reaction of the air entrainment admixture and the ACI.
When using the above-described fly ash activation process or any other process the result is that the activated carbon contaminated fly ash cannot be used to make concrete for use in freeze/thaw zones without massive use of surfactants that can result in overdosing or mix changing that ruins the concrete.
This problem of activated carbon is exacerbated by the increased demand in the use of ACI to treat the flue gas at coal fired power stations due to environmental concerns that mandate removal of mercury from the flue gas which exists from power plant stacks into the atmosphere. This increased useage is a result of increased compliance with environmental standards now in place to reduce mercury contamination in power plant emissions. It is thus important to be able to neutralize the effects of pozzolan or fly ash contaminated with activated carbon to allow its continued beneficial reuse.
More particularly, ACI is injected into the flue gas stream from coal fired boilers in which the carbon reacts with the mercury and captures the mercury from the flue gas. This type of treatment of flue gases has been shown to be effective in capturing all forms of mercury. However, once the mercury has been captured by the activated carbon, the activated carbon flows to the electrostatic precipitator (or bag house) where it is comingled with the fly ash such that the activated carbon is also captured by the electrostatic precipitator or other fine particulate collection system. In any case, the activated carbon ends up in the fly ash and as a result becomes a part of the fly ash.
It is noted that activated carbon or ACI is a very reactive material. It is generally ten times more reactive than carbon produced from the incomplete combustion of coal and thus is a powerful oxidizing material.
Further, as noted above, when activated fly ash is mixed with ordinary Portland cement as a partial replacement, it brings along many beneficial properties when making concrete. A few of these beneficial properties are lower permeable concrete meaning that the concrete is more durable, as a pozzolan it contributes to higher long term strengths than Portland cement alone, and better flow of the wet concrete with lower water to cement ratios and lower slump of flow of the mixed concrete, meaning higher slumps with the same water content.
All of these properties are beneficial to the final concrete and make it a better engineered material with better chemical resistant abilities due to the reduction of the calcium hydroxide that is given off as a waste reaction from the Portland cement by nature of the fly ash pozzolan reacting with the CaOH to form a pozzolanic cement. and the activated pozzolan or fly ash siliceous parts react to form a pozzolanic cement.
However, using activated carbon contaminated fly ash, destroys the ability to entrain air using air entrainment chemicals at normal or elevated levels of doseage.
By way of further background, about 100 million tons of ordinary Portland cement are used in the United States annually and about 25 million tons of fly ash are used in cementitious materials such as concrete. Concrete is the largest building products material used by man today. It is used in almost every structure that is built and is consistently being adjusted to improve its quality.
One aspect that has been corrected over time are the freeze/thaw issues related to concrete.
It was determined that one could entrain air in concrete and thus eliminate the issue of the concrete bursting apart when it is subjected to freezing temperatures and then allowed to thaw. Many concrete highways first paved solely with original Portland cement broke apart completely in areas where freeze/thaw can be a daily or weekly event.
With the advent of air entraining agents (AEA), these surfactants allowed the controlled addition of micro bubbles to be formed in the concrete mix, thus allowing expansion and contraction of the concrete to occur. The result is avoiding the complete breakdown of the concrete. Thus, all modern concrete is treated with air entrainment chemicals to form the protective air bubbles.
It has now been found that the amount of entrained air should be in excess of 4%, but less than 6%.
This being the case, it has recently become apparent that fly ash contaminated with activated carbon causes the air entrainment to be less than desirable due to the highly reactive nature of the activated carbon. The activated carbon, it has been found, can completely soak up the air entraining agents used to produce a safe amount of air in the manufacture of concrete.
To counter the reactivity of the activated carbon, in some cases there have been attempts to increase the dosage of the air entraining agents to offset the activated carbon. The increased dosage of the air entraining agents to provide 4% entrained air now requires 8-10 ounces per cwt such that it becomes almost impossible not to overdose the concrete which results in entrained air in excess of 6% by weight. Note that the overdosing causes strength issues. Moreover, having excessive air results in permeability issues and a host of other problems.
In an effort to remove activated carbon from the fly ash, it has been proposed to eliminate the carbon by combusting it. This can remove the carbon completely and thus yield a pozzolan fly ash that does not have any issues with respect to entrained air. This methodology is very expensive and requires a large area to process and combust the carbon and gives resulting issues in the fly because of the sintering caused when the fly ash is brought up in temperature to combust the carbon.
Other companies such as Separation Technologies and JTM industries use electrostatic separation to eliminate the carbon due to its positive charge. Using positive charge removes most of the activated carbon by forcing the carbon out of the fly ash. This process can remove most but not all of the carbon and it is very sensitive to atmospheric moisture conditions. Thus, there are limits on the production capability associated with electrostatic separation.
As mentioned above, still others have used surfactants or iodine-derived chemicals to attach to the carbon surface and thus lower its ability to impact air entrainment. In one method, these surfactants are applied to the fly ash itself at transfer points utilizing atomized mists. Given the nature of fly ash when the treatment is applied, usually at the power station, the result is that the fly ash is treated and then stored in a tanker to move it to another storage bin. The result is that the fly ash is handled several times. Note, each time the fly ash is handled, usually pneumatically, it is impacted during transfer and the activated carbon that has been surface treated is easily broken down. With the breakdown one has new exposed activated carbon surfaces that again impact air entrainment. Moreover, dosing with a surfactant cannot be exact, as real time carbon injection or formation of unburned carbon cannot be precisely predicted.
The unfortunate result is overdosing that causes huge swings in the entrained air in the concrete, as the non-reacted surfactant, due to overdosing, will cause entirely too much air to form. It is the concrete producer's nightmare that from truckload to truckload the air percentage swings dramatically in and out of specification such that much of the concrete must be rejected.
There is therefore a need for a better method of either removing activated carbon from fly ash or pozzolan or neutralizing it so that the percentage of entrained air can be accurately controlled.